The present invention concerns a method of purifying flue gas deriving from incinerators that burn refuse, wood, or sludge.
The method employs a neutralizer to precipitate the acidic constituents of the gas. Ordinarily the agent will be milk of lime measured out into one or both washers. The products of the neutralization reaction are sluiced in the form of a suspension out of the washers. Crystals of gypsum can be separated from the sluiced-out wash and exploited if desired. The wash or remaining wash is adjusted to a desired pH with more milk of lime in a separate neutralization section and forwarded to a spray drier. The solids that form in the drier as products of the reaction of the wash with the flue gas are sluiced out of the system by way of a dust remover downstream of the drier.
Some particular embodiments of such flue-gas purification plants employ two washers in series, the first using milk of lime to separate hydrohalides and the second sodium hydroxide as a neutralizer to separate sulfur oxides. Laterals from both washers are forwarded to a double-decomposition reactor, wherein gypsum crystals precipitate out of the solution. Final purification of the gas is carried out by a filter-bed adsorber, consisting of a flue-dust reactor with downstream filtering surfaces. Here, a powdered adsorbent is injected into the entering gas and deposits in the form of layers of filtering material on the surfaces. The adsorbent is usually calcium hydroxide with activated charcoal or similar materials added to it to improve its capacity to precipitate heavy metals and organic trace materials. Almost all the adsorbate is returned to the adsorber when the filtering surfaces are cleaned, although some is sluiced out of the system.
In spite of all its advantages in terms of effectively blocking pollutants, the method still has a drawback in that, especially when the washers separate satisfactorily, only a little of the pollutant freight can get into the adsorber and be separated. The adsorbate sluiced out of the system will accordingly still contain more calcium hydroxide than can be used in conventional processing. The excess has to be removed from the system and is accordingly lost.
Another version of the flue-gas purifier that has been employed in practice comprises a spray drier, a dust remover, a nitrogen remover, and a downstream filter-bed adsorber. Milk of lime is employed to neutralize acidic pollutants in the spray drier. The drops emitted inside the drier by the milk-of-lime suspension as it dries will evaporate in the prevailing heat. Neutralization will simultaneously occur in the liquid phase of the drops. Drying leaves the neutralizer and its reaction products in the form of powdered salts, which the dust remover sluices out of the system. Some of these known methods add activated charcoal or coke to the neutralizer to improve the separation of heavy metals and organic trace materials. Final purification of the gas is again carried out with a filter-bed adsorber.
In some flue-gas purifiers, the used adsorbate is taken from the adsorber in the form of dust and returned to the dust remover, downstream of the drier, by way of a conveyer line for further exploitation, especially of the activated-charcoal constituents. Since, however, the conditions upstream of the drier allow only limited reactions between the acidic components of the gas and the lime in the dried adsorbate, much of the lime in the adsorbate goes unused in this alignment as well.
The object of the present invention is a method of the aforesaid genus that will better exploit the lime in the adsorbate.
Processing the adsorbate into an aqueous suspension allows the unreacted lime in the adsorbate to react with the flue gas inside the drier and washers and the acidic components of the gas to separate. The unused lime in the adsorbate can then be used to replace some or all of the milk of lime. This approach will not only decrease the cost of manufacturing the neutralizer but will also reduce the consumption of fossil fuels to manufacture it. Furthermore, reducing the level of unreacted lime will also reduce the amount of waste products subject to disposal regulations. When the suspension containing the adsorbate is introduced into the spray drier, the unused activated charcoal in the adsorbate can also contribute to separating such heavy metals as mercury and such organic trace materials as polyhalogenated aromatic hydrocarbons (PCDD and PCDF).